1. Field of the Invention
This invention relates to a substrate fiber for a dry friction material, to a method of manufacturing the substrate fiber, and to a dry friction material. More particularly, the present invention relates to a substrate fiber that can stabilize the coefficient of friction (xcexc) of dry friction material containing the substrate fiber, to a method of manufacturing the substrate fiber, and to a dry friction material containing the substrate fiber.
2. Discussion of the Background
A conventional dry friction material used in, e.g., a vehicular clutch or brake pad is manufactured by forming and heating a compounded mixture of substrate fiber, a variety of composition materials and thermosetting resins. In the past asbestos has been used as the substrate fiber. Recently, environmental and sanitation concerns have led to aramid fiber as an alternative substrate fiber material.
The engaging and disengaging point of a clutch and the deceleration caused by a brake pad are dependent on the performance of the dry friction material in the clutch and brake pad. Consistent clutching and braking requires that the dry friction material have a stable coefficient of friction (xcexc). This requires that the substrate fiber and the variety of composition materials in the dry friction material be uniformly distributed throughout the dry friction material. This also requires that the uniform distribution of substrate fiber and composition materials be stable regardless of the frequency of engagement of the dry friction material in the clutch or brake.
FIG. 3 shows a dry friction material in which the substrate fiber consists of straight aramid fiber chops 20. However, the ability of straight cylindrical fiber chops 20 to hold composition materials 13 and 14 in the dry friction material is poor. As a result, the adhesion between the substrate fiber and the composition materials (e.g. metallic fiber and son) in the dry friction material is poor. In addition, because the chops 20 do not include divergent portions (e.g., fine fibers) extending from the fiber cylinders, the chops 20 are apt to cohere, so the dispersibility of the substrate fiber chops 20 is poor. As a result, when cylindrical fiber chops 20 are used to form dry friction material, great care must be taken in mixing the chops 20 with other composition materials if satisfactory dry friction material performance is to be achieved. For example, the type of mixer used, the nature of the mixed components, the order of mixing, and the frequency of mixing must all be considered. This makes the manufacture of dry friction material containing cylindrical substrate fiber chops 20 extremely difficult.
FIGS. 2 and 8 show one approach to avoiding the difficulties presented by cylindrical substrate fiber in forming dry friction material. In FIG. 2 a fiber pulp 10 comprises aramid fibers each having a trunk portion 11 with a diameter of about 0.1 to several xcexcm and a length on the order of 2 mm. In this pulp 10 numerous divergent portions 12 with diameters on the order of nm to several xcexcm diverge from the trunk portion 11. The divergent portions 12 enhance the dispersibility of the fiber pulp 10, so the fiber pulp 10 can be distributed uniformly. This makes it relatively easy to mix the fiber pulp 10 in a dry friction material.
However, the fiber pulp 10 can make it difficult to mix various other composition materials in a dry friction material. Composition material 13, which is smaller than several xcexcm, can be distributed fairly uniformly in the small spaces left by the divergent portions 12 of the fiber pulp 10. However, composition material 14, which is larger than several tens of xcexcm, cannot be distributed uniformly in the fiber pulp 10 and tends to cohere. As a result, dry friction materials containing fiber pulp 10 do not contain a uniform distribution of composition materials.
When dry friction material having a non-uniform distribution of substrate fibers and composition materials is used repeatedly, a large number of divergent portions which are diverged from the substrate fibers subsist and tend to appear at the periphery of the frictional surface of the dry friction material. As a result, the condition of the frictional surface changes in response to the frictional history and the coefficient of friction (xcexc) of the surface becomes unstable.
The present invention provides an improved substrate fiber for a dry friction material that allows the substrate fiber and other composition materials in the dry friction material to be dispersed equally or uniformly in the dry friction material. Each substrate fiber includes a trunk portion extending in the axial direction of the fiber. The trunk portion fits within a maximum diameter of from about 10 to 20 xcexcm. The length of the trunk portion is from about 0.5 to 5 mm. Slightly diverging from a part of the trunk portion is at least one divergent portion having a small diameter less than the maximum diameter. The divergent portions in the substrate fibers prevent the fibers from cohering, while allowing the substrate fiber and the composition materials to be uniformly dispersed in the dry friction material.